This relates to a modulator/demodulator, or modem, used to transmit and receive data in a data communication system and more particularly to a method and apparatus for operating wide band modems to provide high speed data transmission in a communication network.
As is well known, data is often transmitted over a communication line in the form of pulses of certain frequencies. At one station in the network, the modulator in the transmitter portion of a modem is used to convert a DC signal representative of a stream of digital data into an AC signal representative of this same stream of digital data. At another station in the network, a demodulator in a receiver portion of a second modem converts received AC signals back to digital DC signals. Ordinarily, data communication takes place in both directions on the communication line and each modem is equipped both to convert DC signals to AC signals that are transmitted and to convert received AC signals to DC signals.
In a data communication system using frequency shift keying (FSK), one of the two DC levels that represents digital data is converted by the modem to an AC signal having a first frequency; and the other level of the DC signal is converted to an AC signal having a second frequency. It is conventional in the art to refer to one of these DC levels and the corresponding AC frequency as a SPACE or "0", and to the other DC level and the corresponding AC frequency as a MARK or "1".
Modems are used in many types of data communication systems. Of particular interest to the present invention are the wide band modems used for high speed data transmission in a distributed processing system. In these systems modems are used to couple individual stations to a common coaxial line. Each station includes a data source, a data sink, or both. For example, the station might include a Teletype terminal, a keyboard, an intelligent terminal, a microcomputer, or a central processing unit. Because of the speed at which the computers in such a system are able to generate and transmit data, it is desirable to interconnect the individual stations by modems and transmission lines which are capable of high speed communication. In particular, it is desirable to use wide band high frequency modems and transmission lines.
To minimize signal losses in transmitting high frequency signals, it is necessary in such systems to interconnect the individual stations by a coaxial line. The use of such a line, however, raises additional problems. Typically, each station is connected to the line via a simple T-coupler such that the signal amplitude is split in half at each coupler. Obviously, this places severe constraints on the number of stations that may be coupled in a single system. Other restrictions are imposed on the minimum distance between stations.